The composition of the interfacial system between the composite-modified double-base solid propellant grain and insulation in a missile motor is generally comprised of an embedment powder and an embedment resin which comprises an embedment system. Compatibility between the grain composition and the embedment system is necessary since this system functions as an interface between the propellant grain and the insulation system. Requirements for the embedment system include a high peel strength and a resistance to penetration by, or absorption of, catalyst components or casting solvent components contained in the propellant composition. Without proper peel strength and resistance to penetration the bond strength to the propellant and insulation is adversely effected. Failure of the bond in the embedment system would result in a sudden major increase in the burning surfaces of the propellant, and possible rupture of the motor case due to overpressurization. The burning rate of the propellant grain would also be adversely affected as well as the smooth burning rate because of poor adhesion to the embedment system.
An embedment system for use in conjunction with a crosslinked, carboranyl-catalyzed, composite-modified, double-base propellant composition is designed to have as close a similarity to the propellant as possible from compatibility considerations but still must differ so that its burning rate is as low as possible (approaching zero burning) as a consequence the embedment layer differs from the propellant composition with which it is used in the following aspects: the embedment powder contains no carboranyl burning rate accelerator, no aluminum whiskers, and the particle size of the ammonium perchlorate is of considerably larger weight-mean-diameter. The other portion of the embedment system is an embedment resin portion composed of the diglycidyl ether of bisphenol A (EPON 828) and epoxidized Dimer* acid (EPON 871). (Emery Industries of Cincinnati, Ohio is a major supplier of dimerized oleic acid which is marketed as Dimer* acid.) The curative presently used consists of a blend of meta-phenylenediamine, methylenedianiline, isopropyl meta-phenylenediamine, and dinitrophenol as accelerator. This system is a diamine-cured epoxy embedment system.
The above described embedment system has several limitations and has proven to be unsatisfactory because of lower than required peel strength and a lower resistance to penetration by, and absorption of, the catalyst and the casting solvent from the propellant composition.
The polyimide embedment system for composite-modified double-base propellant disclosed in my U.S. Pat. No. 4,441,942, filed Jan. 3, 1983 and issued Apr. 10, 1984, has superior characteristics when compared to the characteristics of a diamine-cured epoxy embedment system. The polyimide embedment system of the above U.S. Patent, assigned to The United States of America as represented by the Secretary of the Army, Washington, D.C., is comprised of an embedment granule portion in combination with a substituted polybismaleimide prepared by the interaction or condensation of 2 moles of 1,2-bis(maleimido)ethane with one mole of triaminotriazine(melamine) at 80.degree.-100.degree. C. to yield the low-molecular weight reaction prepolymer (e.g., molecular weight range=2000-2500). This prepolymer, when incorporated into the embedment resin matrix, reacts with epoxy resins. The embedment system when cured has improved peel strength and greater resistance to penetration by or absorption of carboranylmethyl propionate (catalyst) or casting solvent absorption, however, the shrinkage of the resin when cured detracts from the embedment system mechanical properties.
An object of this invention is to provide an embedment system with controlled shrinkage for use with a composite-modified double-base propellant composition.
A further object of this invention is to provide shrinkage control by incorporating a bisspiro orthocarbonate into the embedment system composition which results in an expansion during curing to offset shrinkage normally encountered when resins undergo curing.